1) Field of the Disclosure
The disclosure relates generally to composite structures, and more specifically, to composite radius fillers for use in composite structures, such as in aircraft, and methods for forming the same.
2) Description of Related Art
Composite structures, such as carbon fiber-reinforced plastic (CFRP) composite structures, are used in a wide variety of applications, including in the manufacture of aircraft, spacecraft, rotorcraft, watercraft, automobiles, trucks, and other vehicles and structures, due to their high strength-to-weight ratios, corrosion resistance, and other favorable properties. In aircraft construction, composites structures are used in increasing quantities to form the fuselage, wings, tail sections, and other components.
For example, aircraft wings may be formed of composite stiffened panel structures comprising composite skin panels or webs to which reinforcing stiffeners or stringers may be attached or bonded to improve the strength, stiffness, buckling resistance, and stability of the composite skin panels or webs. The reinforcing stiffeners or stringers attached or bonded to the composite skin panels or webs may be configured to carry various loads and may be provided in a variety of different cross-sectional shapes, such as T-stiffeners, J-stiffeners, and I-beams. To assist the load carrying capability of the wing, a series of ribs may be connected to the stringers using shear ties. FIG. 4A is an illustration of a perspective view of a known shear tie and monolithic rib assembly 68 for an aircraft wing 18 (see FIG. 1). FIG. 4A shows monolithic ribs 70 with shear ties 72 interfacing with stringers 74 and skin panels 76. FIG. 4B is an illustration of a perspective view of a known shear tie and airload rib assembly 78 for an aircraft wing 18 (see FIG, 1). FIG. 4B shows shear ties 80 with ribs 82.
Gaps or void regions may be formed by the radius of each curved piece of the reinforcing stiffeners, such as T-stiffeners, J-stiffeners, and I-beams, when they are attached or joined perpendicularly to composite skin panels or webs. Such gaps or void regions may typically be referred to as “radius filler regions” or “noodle regions”. Such radius filler regions or noodle regions within reinforcing stiffeners may be prone to cracking because they may be three-dimensionally constrained. Radius filler elements or “noodles” made of composite material or adhesive/epoxy material and having a generally triangular cross-section may be used to fill the radius filler regions or noodle regions in order to provide additional structural reinforcement to such regions.
Known configurations of radius filler elements or noodles exist. For example, such known configurations of radius filler elements or noodles may include CFRP radius filler elements or noodles that are extruded and bundle all zero degree plies with unidirectional fibers. However, such extruded all zero degree ply CFRP radius filler elements or noodles may have high through-thickness thermal expansion and resin shrinkage that may lead to high residual stresses, i.e., internal stresses created inside a component during manufacturing, such as thermal residual stress that may be created during heat curing. In addition, the unidirectional fibers of such extruded all zero degree ply CFRP radius filler elements or noodles may have low pull-off strength and may pull apart as a result of high residual stresses that may be created during heat curing at high temperatures, i.e., such as 350 degrees Fahrenheit or greater, and subsequent exposure to cold temperatures, i.e., such as less than −65 (minus sixty-five) degrees Fahrenheit, which may, in turn, lead to stress or fatigue cracking in the CFRP radius filler elements or noodles. To decrease the likelihood of such stress or fatigue cracking due to low pull-off strength and high pull-off loads, the use of shear ties on the wing ribs may be required. However, the use of such shear ties may add weight to the aircraft due to the possible need for a shear tie at each location where a rib intersects with a stringer. The added weight of the shear ties at each rib-stringer intersection may reduce the payload capacity of the aircraft and may increase fuel consumption and fuel costs. In addition, the addition of a shear tie at each rib-stringer intersection may increase manufacturing complexity, cost, and production time.
In addition, known laminated radius filler elements or noodles exist that have a generally triangular cross-section and that are constructed using a pyramid of plies in a single direction. However, such known laminated radius filler elements or noodles may minimize residual thermal stresses at only two points or peaks of the known laminated radius filler element or noodle but not at all three points or peaks of the known laminated radius filler element or noodle.
Accordingly, there is a need in the art for improved composite radius fillers and methods of forming the same that provide advantages over known elements, assemblies and methods.